I have been using OS X for nearly a year now but I never bothered to find out how to use the built-in Dictionary properly until a few weeks ago. It is actually surprisingly useful. So a quick heads up for those who have never used it.
You can type any word in to Spotlight and Dictionary will offer a definition for it. You can also open Dictionary through the Applications folder.
There are various dictionaries available, depending on which languages you use, and you can select the ones you want in Dictionary > Preferences. There’s also a Thesaurus and an option for Synonyms. You can also get definitions straight from Wikipedia but you have to be online to get them.
You can change the order in which definitions appear by changing the order in which they are listed here.
I find Dictionary particularly useful for translations. It is very useful as it will translate from Japanese to English
and from English to Japanese.